This invention relates to novel ester compounds which are useful as a monomer to produce base polymers for use in chemically amplified resist compositions for microfabrication.
In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography. Also, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation. Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.30 micron process, went through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated.
For ArF laser (193 nm), it is expected to enable miniaturization of the design rule to 0.13 xcexcm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198.
With respect to F2 excimer laser (157 nm) which is expected to enable further miniaturization to 0.10 xcexcm or less, more difficulty arises in insuring transparency because it was found that acrylic resins which are used as the base resin for ArF are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. It was also found that poly(vinyl phenol) which is used as the base resin for KrF has a window for absorption in proximity to 160 nm, so the transmittance is somewhat improved, but far below the practical level.
An object of the invention is to provide a novel ester compound useful as a monomer to produce a base polymer which has a high transmittance to vacuum ultraviolet radiation of up to 300 nm, especially F2 (157 nm), Kr2 (146 nm), KrAr (134 nm) and Ar2 (121 nm) excimer laser beams, and is useful as the base resin in a chemically amplified resist composition.
It has been found that resins obtained from acrylic acid derivatives containing fluorine at xcex1-position have a high transparency and that resins synthesized from the same monomer, but having a lactone ring introduced into the ester side chain thereof is outstandingly improved in adhesion to substrates.
The present invention provides an ester compound of the following general formula (1): 
Herein R1 is a fluorine atom or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms having at least one fluorine atom, R2 is hydrogen, a fluorine atom or a straight, branched or cyclic alkyl or fluorinated alkyl group of 1 to 15 carbon atoms, R3 and R4 each are a single bond or an alkylene group of 1 to 20 carbon atoms.
The present invention also provides an ester compound of the following general formula (2): 
Herein R1 is as defined above, and R5 is an alkylene group of 1 to 10 carbon atoms, an oxygen atom or a sulfur atom.
Preferably, R1 in formula (1) or (2) is trifluoromethyl.